I don't know what you mean by "Line sensor." Do you mean like a pencil line on a piece of paper? One thing you could do is get a cheap light sensor( such as this one: http://www.plazaearth.com/usr/gasperi/dlite.htm) and just program it to detect the change in brightness.

valop,great to read your mesg. my major is automaiton and I also have a document about robot. please contact with me, I will send it to you!
regard!!!!_________________the sword conquers for a while, but the soul conquers forever!